Elasticsearch Alerting and Monitoring in versions before 6.4.1 or 5.6.12 have an information disclosure issue when secrets are configured via the API. The Elasticsearch _cluster/settings API, when queried, could leak sensitive configuration information such as passwords, tokens, or usernames. This could allow an authenticated Elasticsearch user to improperly view these details.
https://www.elastic.co/community/security
https://discuss.elastic.co/t/elastic-stack-6-4-1-and-5-6-12-security-update/149035